1. Field of the Invention
The present invention relates to a novel electromagnetic wave absorption material and associated devices. The invention relates to various articles particularly, such as, an electromagnetic wave absorption material for millimeter wave of 1 to 300 GHz band, a printed wiring board that uses such absorbing material, an electronic device, a casing for an electronic device, a module for optical transmission or reception, automated tollgate, and high frequency communication equipment.
2. Description of the Prior Art
In late years, the high speed processing in electronic devices is spreading at an accelerating pace together with rapidly raised operating frequency of ICs and LSIs in microprocessors. This raised frequency allows such devices to easily emit undesired noise.
Moreover, in the telecommunications field, the next-generation multimedia mobile communication (on 2 GHz), the wireless LAN (on 2 to 30 GHz), and the high-speed telecommunication network on the optical fiber are presently in use. Further, 5.8 GHz in ETC (an automated Electronic Toll Collection) system in ITS (Intelligent Transport System), 76 GHz in AHS (an advanced cruise-assist highway system) are also in use. Therefore, a rapid expansion of use-range toward higher frequencies is continuously anticipated.
Rise in frequency of a radio wave causes more eased emission in a form of noise. At the same time, narrowed noise margin in the recent electronic device because of lowered power consumption therein, together with poor quality in noise-ambient inside an apparatus because of miniaturization and dense mounting in the electronic device, brings a malfunctioning problem due to EMI (Electro-Magnetic Interference).
Under these circumstances, a measure such that a radio wave absorbing material is installed inside an electronic device is typically adopted to reduce EMI that will appear inside the device. One of radio wave absorbing materials so far used is a composite sheet of electrically insulating organic matters, such as rubber and resin, and magnetically lossy material, like soft magnetic metal oxides having spinel crystal structure and soft magnetic metals. These techniques were disclosed by Japanese Patent application laid-open No. 7-212079, 9-111421, 11-354973, 11-16727, etc.
However, the relative permeability of a soft magnetic metal oxide of spinel crystal structure sharply reduces in the GHz-band according to the Snoek""s law of threshold This means that the usable frequency range for such metal oxide as an electromagnetic wave absorption material is up to several-GHz. In a soft magnetic metal on the other hand, its marginal frequency usable as a electromagnetic wave absorption material can be expanded up to about 10 GHz by use of eddy current suppression effect and shape-dependent magnetic anisotropy effect both yielded from making the particle therein into a flat-shape having a thickness thinner than the skin depth. However, such magnetic material prevents realization of light-weight electromagnetic wave absorber because of its inherent weightiness.
Japanese Patent application laid-open No. 2001-358493 has disclosed an integrated electromagnetic wave absorption material composed of particulate magnetic metal and ceramic, and those articles or facilities, which use said integrated absorption material, such as a printed wiring board, an electronic device, a casing for electronic device, an optical transmission or reception module, an automated tollgate, a high frequency telecommunication equipment, etc.
However, said magnetic material is still unsatisfactory in its electromagnetic wave absorption performance. Thus, development of an electromagnetic wave absorption material having an excellent performance in the extended use up to millimeter wave region is continuously desired. Although the electromagnetic wave absorption material according to said Japanese Patent application laid-open No. 2001-358493 is satisfactory for sub-millimeter wave region up to 5.8 GHz, the use in the millimeter wave region over such frequency does not satisfy with the performance of said material.
The purpose of the present invention is to provide an electromagnetic wave absorption material having an excellent radio wave absorbing performance usable spanning from sub-millimeter wave region to millimeter wave region in which radio wave frequency ranges from 1 to 300 GHz, and the one being easy to manufacture and light in weight. The purpose further includes to provide various appliances of and systems with said electromagnetic wave absorption material particularly an electronic device, an optical transmission or reception module, a high frequency telecommunication equipment, and a stop-free tollgate system which is prevented from malfunctioning due to electromagnetic wave interference.
The inventors of the present invention found that an electromagnetic wave absorption material comprised of a dispersions of multi-layer hollow globule of carbon mixed into electrical insulating organic material has a far more excellent performance as the electromagnetic wave absorption material available for use in millimeter wave region compared to an electromagnetic wave absorbing material relying on dielectric loss, i.e. a dispersions of carbon-based substance, such as carbon black particulate, graphite, coke, carbon microcoil, and carbon nanotube, mixed into electrical insulating organic material like rubber and resin. Since said multi-layer hollow globule of carbon or a multi-layer hollow globule of carbon existing in a natural schungite ore (hereinafter referred to as a schungite carbon) is contained in natural schungite ore, the use such material for the electromagnetic wave absorption material sees little difficulty. Particularly, the present invention is devised based on the finding that such globule has a high absorption property in millimeter wave region of which frequencies are 30 to 300 GHz.
The schungite carbon for this purpose is preferred to have 1.5 to 45% of porosity, 0.15 to 0.25 nm of thickness, and to have a shape being spherical or flat, or their mix. As for the natural schungite ore, preferred contents is 25 to 35% of the schungite carbon, 57 to 67% SiO2, 3 to 5% AlO3, 1 to 3% Fe2O3+FeO, 0.5 to 2% K2, 0.2 to 1.0% sulfur, 0.2 to 0.5% free water, and 0.3% or less for each of TiO2, MgO, CaO, Na2O, and MnO.
The present invention is characterized by an electromagnetic wave absorption material that includes: one of a multi-layer hollow sphere (globule) of carbon and a schungite carbon; or one of a multi-layer hollow sphere and a schungite carbon, said multi-layer hollow sphere containing at least one of a carbon nanotube, a metallic component, and free water; or a schungite ore otherwise.
The electromagnetic absorption material in the present invention is preferred to be a dispersions mixed into a substance that has a higher electrical resistance than that of multi-layer hollow globule or the schungite carbon. In this dispersing, the quantity of the multi-layer hollow globule and the schungite carbon is preferred to be in a range of 5 to 50% of the weight of such high-resistance substance. This high-resistance substance is preferred to be selected among from rubber, insulation high polymer, and insulating inorganic material.
The electromagnetic wave absorption material in the present invention prefers a configuration wherein content of the multi-layer hollow globule or the schungite carbon is graded to plural degrees so that characteristic impedance may reduce toward inside from the incident plane of electromagnetic wave. Thereby, the electromagnetic wave absorbing performance for an oblique incidence of electromagnetic wave is improved or becomes being capable of accommodating electromagnetic wave of wide range of frequencies.
Material having a high electrical resistivity can contain at least one of: a magnetic metal of which main constituent is at least one of iron (Fe), cobalt (Co), and nickel (Ni); a compound selected from the group consisting of an oxide, a nitride, and a carbide, in which their main constitution is at least one of iron (Fe), cobalt (Co), nickel (Ni), aluminum (Al), silicon (Si), titanium (Ti), barium (Ba), manganese (Mn), zinc (Zn), and magnesium (Mg); and a carbon-based substance which contains at least one of carbon black, graphite, coke, and carbon microcoil. Thereby the absorbing performance of the electromagnetic wave absorption material is more improved.
The present invention is characterized by an electromagnetic wave absorption material comprising an electromagnetic wave absorbing particulate and a substance that has an electrical resistivity higher than said electromagnetic wave absorbing particulate, wherein the return loss (reflection coefficient) of said electromagnetic wave absorption material is xe2x88x923.5 dB or more (in absolute value) at a radio wave frequency of 1 GHz and the return loss of the same is xe2x88x9220 dB or more at a radio wave frequency of 6 GHz. Said electromagnetic wave absorbing particulate prefers to be comprised of said electromagnetic wave absorption material. Such return loss levels can be attained by regulating combination of thickness of said electromagnetic wave absorption material and content of said electromagnetic wave absorbing particulate in a composite.
The electromagnetic wave absorption material by the present invention can be applied to a broad range of articles typically listed below through various suited methods depending on each article. Said absorption material is applied in a form of dispersions mixed in said insulating material for coating or for sheeting to glue, or in a style of a near net shape by injection molding.
(1) A printed wiring board in which all or a part of at least one of surfaces of said wiring board, one wired surface and the other the reverse surface thereof which has no circuit wiring, is covered with a layer of direct coating or a sheet-formed film each comprised of the electromagnetic wave absorption material by the present invention.
(2) An electronic device in which an electronic element mounted thereon is covered with a cap that has the electromagnetic wave absorption material by the present invention.
(3) An electronic device in which a printed wiring board thereof and an electronic element mounted on said printed wiring board is covered with a casing having the electromagnetic wave absorption material by the present invention, or in which the internal surface of a metal casing having an opening is provided with the electromagnetic wave absorption material by the present invention.
(4) A module for optical transmission or optical reception in which at least either a light emitting element thereof or a photo acceptance element thereof, each of which has an opt-electric conversion device for use in a high-speed telecommunication networks, and at least either a transmission circuitry therein or a reception circuitry therein are covered with the member that is equipped with the electromagnetic absorption material described by the present invention.
(5) An automated tollgate comprising a roofed tolling gate, an entrance antenna arranged at the entrance of said tolling gate and faced to a transit vehicle approaching said tolling gate, an exit antenna arranged at the exit of said tolling gate and faced to a transit vehicle leaving said tolling gate, and an automated toll collection system that exchanges information between a roadside communication equipment and an on-vehicle equipment carried by said transit vehicle, wherein the surface of a structural member in said tolling gate and its vicinity which reflects electromagnetic wave, the surface of facing-to-vehicle side of the roof of said tolling gate, and at least a part of the surface of a supporting column for said entrance antenna and said exit antenna are provided with the electromagnetic wave absorption material described by the present invention.
(6) A high frequency communication equipment comprising a mounted high frequency circuit element and an antenna both accommodated inside a casing, wherein at least the inner wall of said casing, and at least a part said inner wall, are provided with the electromagnetic wave absorption material described by the present invention.
According to the present invention, it is practicable to provide an economical electromagnetic wave absorption material having far more excellent performance in absorbing property than that in a dielectric loss type electromagnetic wave absorbing material in the prior art comprised of carbon-based material.
Further according to the present invention, electromagnetic wave interference within an electronic device is efficiently suppressed by use of a light weight electromagnetic wave absorption material having an absorbing property usable from submillimeter wave region to millimeter wave region. Thus, it becomes practicable to provide equipment acceptable for use in a high speed telecommunication network, such as semiconductor device, an optical transmission module, an optical reception module, or an optical transmission-reception module, and high frequency telecommunication equipment; thanks to their capability rendered from the absorbing material of suppressing internal noise interference and noise emission to outside, of achieving small-sizing and weight-reduction, of working under high speed transmission, and of having high sensitivity.
Moreover according to the present invention, it also becomes practicable to provide an automated tollgate capable of exchanging information between a roadside communication equipment and an on-vehicle equipment free from suffering electromagnetic wave interference.